This invention relates to surface treated titanium and titanium alloy articles having a thin anodized film substantially of TiO.sub.2 and characterized by having low leakage current, high dielectric strength, high breakdown potential, and high corrosion resistance This invention also relates to the process for forming such titanium and titanium alloy articles with the process being characterized by anodizing the articles in a substantially non-aqueous solution of a mineral acid and an organic solvent.
Titanium metal and its various alloys have two primary and significant characteristics of commercial interest, namely: high structural efficiency, and high corrosion resistance in oxidizing environments. Because of its high structural efficiency titanium metal and its alloys have had numerous aerospace applications. The high corrosion resistance of titanium and its alloys have rendered them useful in various chemical processing applications. Corrosion applications depend on the existence of a passive film of TiO.sub.2 on the surface of the metal. Exposure of the metal to moist air or oxidizing aqueous media are sufficient to establish a passive film. This naturally occurring film is the basic reason why titanium is corrosion resistant in oxidizing media at ambient to the moderate temperatures used in processing aqueous media.
Pure TiO.sub.2 also has high dielectric properties. However, its dielectric properties have been heretofore not extensively taken advantage of, mainly because thin films of TiO.sub.2 created by known anodizing methods have been less efficient in preventing current leakage in the presence of an electrical field, as compared, for example, to Ta.sub.2 O.sub.5 or Al.sub.2 O.sub.3. The leakage current, as it is known, is that current that still flows across a film in response to an electrical field after anodization is completed. TiO.sub.2 has found extensive use as a constituent in mixtures with other oxides in passive electronic devices such as ceramic capacitors, but has not had any known use as a pure oxide or anodized film.
Titanium may be anodized in a variety of aqueous solutions compromised of acids, bases, or salts. None of the known methods of anodizing TiO.sub.2 films result in articles being produced where leakage currents are below 25 microamps per square centimeter. Dilute aqueous solutions of boric acid solutions permit anodization to high voltages but the leakage currents are also very high. Titanium has also been anodized in aqueous solutions of methyl ethyl phosphate to about 350 volts, but resulting oxide typically produces leakage currents about 40 microamps per square centimeter at about 200 volts. Other methods of anodizing titanium have been known such as that disclosed in U.S. Pat. No. 2,874,102 where titanium is disclosed to be anodized to a "desired maximum value". However, the electrolytes disclosed are significantly inefficient since they give rise to an electrically leaky oxide. Other attempts at anodizing titanium such as anodizing in fused-salt baths but have met with only partial success. The use of molten nitrate electrolytes at 300 degrees C or higher prove to be impractical and in some instances dangerous and the attempts at fused-salt anodizing where abandoned.